The summer movie season gets off to a raucous start with Marvel’s superhero extravaganza “The Avengers.” It’s not just comic book geeks who are frothing at the mouth with excitement over the reunion of this elite squadron. Marvel’s been building anticipation ever since 2008’s “Iron Man” started raking in the big bucks ($585 million worldwide), and the success of “Iron Man 2,” “Captain America” and “Thor” only added to the anticipation for the moment when the complete Avengers team could take over the cineplex.

A swift setup puts an otherworldly, limitless power source (the mysterious cube seen in the teaser at the end of “Thor”) in the hands of Loki (Tom Hiddleston), the banished, power-hungry brother of Thor (Chris Hemsworth), who plans to use it to defeat and subjugate the entire human race. Since the all-powerful cube was in the hands of S.H.I.E.L.D., the secret global organization that acts as the planet’s protector, it falls to its director, Nick Fury (Samuel L. Jackson), to find a way to get it back.

While S.H.I.E.L.D. had already scrapped the idea for an all-star superhero squad due to the unlikelihood that they would play nicely with one another, Fury knows that Loki’s threat is too great and calls in the scattered team, many of whom would rather be left out of it. Fury dispatches one of his top agents, Natasha Romanoff (Scarlett Johansson), a skilled assassin known as Black Widow, to persuade Dr. Bruce Banner (Mark Ruffalo) to stop hiding from his monstrous inner Hulk and join the scientific effort to track down the missing energy source. Also reluctantly on board are billionaire playboy philanthropist and Iron Man Tony Stark (Robert Downey Jr.); Captain America (Chris Evans), in seclusion after finding himself alive during an American era he can’t respect; and Hawkeye (Jeremy Renner), who has superb skills with a bow and arrow.

Getting the gang back together again requires most of the film’s first act. Writer/director Joss Whedon (“Cabin in the Woods,” TV’s “Buffy the Vampire Slayer”) creates some genuine fun as these oddities get into one another’s faces and reveal their superhero insecurities, but his signature wry character-building gets diluted by the sheer number of characters to be introduced and tracked throughout the 2½-hour film. This is especially true with Black Widow and Hawkeye, who didn’t get stand-alone films to introduce them to audiences.

Whedon also encounters a few pacing problems along the way, sometimes losing track of one story arc while delving into another, or by too often cramming large amounts of exposition into lengthy scenes of flat dialogue. If the plot were that complicated, perhaps it would be justified — but it’s not. Whedon’s time would have been better spent unwrapping more of the characters’ stories and rocky relationships.

Not surprisingly, it’s the same characters that bowled over audiences in summers past that carry the charm “The Avengers” requires to appeal to people outside the Comic-Con sphere: the cocksure Tony Stark and the ultra-honorable World War II hero Captain America. While Mark Ruffalo’s version of Dr. Banner/The Hulk is new to the big screen, he makes a smashing first impression and easily has the best comic moments. A future partnership between the gentle ticking time bomb Banner and the smart-aleck Stark would be most welcome.

Alas, “The Avengers” is not just a reunion special; there’s a war to be won. Unfortunately, Loki doesn’t come across as all that threatening, and instead seems more like a seriously misguided, sniveling little brother. While his alien allies are more fearsome, the film and its heroes carry with them a sense of inevitability that undermines the threat. If watching this dream team strut their stuff in some epic clashes is what you’ve signed up for, then you won’t be disappointed.

It’s also nice to see 3-D put to good use, giving “The Avengers” the near-tangible quality of a comic book, with outlandish crashes and combat scenes that seem to jump out of the panels on the page or, in this case, the screen. It’s all rather serious and ominous, yet outlandish and mostly entertaining. Sounds about right for summer, doesn’t it?

Alison Gang is the U-T’s movie critic. Email her at alison@alisongang.com